I saw this in the Guardian today:
Start running and watch your brain grow, say scientists
Neuroscientists at Cambridge University have shown that running stimulates the brain to grow fresh grey matter and it has a big impact on mental ability.
A few days of running led to the growth of hundreds of thousands of new brain cells that improved the ability to recall memories without confusing them, a skill that is crucial for learning and other cognitive tasks, researchers said.
The new brain cells appeared in a region that is linked to the formation and recollection of memories. The work reveals why jogging and other aerobic exercise can improve memory and learning, and potentially slow down the deterioration of mental ability that happens with old age.
Shame they weren't looking at resistance training too.
Here is the Telegraph version